This invention relates generally to the field of collapsible reusable shipping containers of a type disclosed in my prior U.S. Pat. Nos. 3,358,049, granted Apr. 24, 1961; 4,411,373, granted Oct. 25, 1983; 4,231,149, granted Dec. 16, 1981; 4,252,266, granted June 7, 1980; and 4,405,077, granted Sept. 21, 1983. More particularly, the invention relates to improved constructions having superior stacking capabilities both in erected and collapsed state.
Such containers are characterized generally in the provision of a pallet-like "slip sheet" which replaces the conventional wood pallet, an open top box element, the sides of which are foldable to flattened condition to lie upon the slip sheet, a lid or cover element having short sides is then positioned upon the collapsed sides of the box element to form a generally rectangularly shaped planar unit several inches high which is capable of being stacked for storage or empty return shipment. Where additional compression strength is required, a collapsible liner element is provided to provide side walls of greater effective rigidity.
In erected condition, the lid or cover element is secured to the side walls using expandable clip elements which penetrate aligned openings in the cover element and walls and the end walls of the box element.
In my prior U.S. Pat. No. 4,405,077, there is disclosed an improved foldable side wall construction which enables the corner edges of all side walls to be rapidly collapsed and erected, and in which each corner as it is folded overlies an adjacent side in serial fashion. While convenient, this particular construction has been vulnerable to bulging in an upward direction under the influence of the resilience of the folded liner element which is stored in an uncollapsed lower portion of the box element. This bulging effect is transmitted to the cover element resulting in the upper surface of the cover element being moved from a horizontal plane causing collapsed containers position thereon to be tilted and slide from stacked condition. The bulging effect is cumulative, with each stacked unit, this effect severely reducing the stacking capability in collapsed condition.
Another serious problem with stacked containers when in erected and loaded condition is the compressive strength in a vertical direction, particularly at the corners of the container. The greater the compression strength the greater is the number of erected loaded units that may be stacked in a single column. To improve this aspect, it is known to provide a foldable reinforcing element, as disclosed in my U.S. Pat. No. 4,411,373, above mentioned. In certain applications, it is desirable to have a reinforcing element having compressive strength greater than is possible with the disclosed construction.